Problem: Solve for $a$ : $4 = 5 + a$
Answer: Subtract $5$ from both sides: $ \begin{eqnarray} \\ \hphantom{4 {- 5}} &\hphantom{=}& \hphantom{a} \\ 4 &=& 5 + a \\ \\ {-5} && {-5} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 4 {- 5} &=& a \\ \end{eqnarray} $ Simplify. $ a = -1$